This invention relates to block copolymers of ethylene and cyclic siloxane monomers which are made by anionic polymerization. The invention also relates to an anionic process for making such block copolymers.
Linear block copolymers of polystyrene and polydimethylsiloxane have been synthesized, both by graft and block copolymerization. In block copolymerization of such linear polymers, polystyrene is produced by anionic polymerization with an organo lithium initiator and the living polymer (PS.sup.- Li.sup.+) created thereby is reacted with hexamethylcyclotrisiloxane, (Me.sub.2 SiO).sub.3, in the presence of a polar promoter wherein a block of polydimethylsiloxane grows on the end of the living vinyl aromatic hydrocarbon polymer block. These polymers are useful for impact modification of engineering thermoplastics and forming coatings with low energy surfaces.
Graft copolymers of polyethylene and polydimethylsiloxanes have been produced. They comprise a soft block of vinylmethylsiloxane and dimethylsiloxane monomer units. Polyethylene blocks grafted by free radical copolymerization of ethylene with the pendant vinyl groups of the polysiloxane or by Ziegler-Natta copolymerization of ethylene with the pendant vinyl groups as described in Japanese published patent application JP 05032830A and Makromol. Chem., 190(10), 2373-80. However, both synthesis techniques can be expected to lead to polymerization of significant quantities of homopolyethylene in parallel to the desired graft polymerization due to extensive chain termination and chain transfer reactions which are characteristic of these two polymerization processes. Thus, these methods have the disadvantages of producing impure products, products of relatively high molecular weight (hence, much higher solution and melt viscosities) and products with structures that are not well-defined.
Linear diblock polymers of polyethylene (PE) and polydimethylsiloxane (PDMS) have also been produced as described in British Patent No. 1376446. These materials were prepared by sequential anionic polymerization of butadiene and (MeSiO).sub.3 (D.sub.3) to give polybutadiene (PBD) - PDMS block copolymers. However, these materials have the disadvantage that the PBD-PDMS precursors must by hydrogenated to give PE-PDMS polymers. A hydrogenation catalyst removal step is then required and thus four process steps are required to obtain the desired PE-PDMS copolymers. They suffer from the further disadvantage that the resulting PE blocks are low density polyethylene (LDPE) because the anionic polymerization of butadiene is known to give 6-8% branching in the polymer backbone (hence, 6-8% comonomer and thus LDPE). This limits the crystallinity and melting point (M.P.) of the PE block in the PE-PDMS block copolymer and thus limits the strength of the materials and their upper use or service temperatures.
Thus, it can be seen that it would be advantageous to be able to produce a block copolymer with saturated blocks without hydrogenation. It also would be advantageous to be able to produce such a polymer with blocks that exhibit the high level of crystallinity and high melting point of high density polyethylene and thus have higher strength and service temperature capability than the (PBD) - PDMS copolymers discussed above. Finally, it would be advantageous to produce such polymers that are relatively pure and low in molecular weight and have well-defined structures (graft or comb polymers vs. precise linear diblocks, triblocks, or radial polymers of this invention). The present invention describes a process which meets the needs discussed above and produces a polymer with the desired characteristics.
Ethylene has long been known to polymerize anionically as described in Journal of Applied Polymer Science, Vol. 42, 533-41 (1991) but many difficulties and limitations of this process for making useful materials have been reported. For example, at a degree of polymerization of about C.sub.40, the growing living PE-Li chains precipitate from solution and seriously retard the rate of polymerization. Furthermore, the mole weights of the PE so obtained generally plateau at about 2000 to 3000 and it is only with great difficulty that MWs in the 5000 to 8000 range can be obtained effectively limiting the practical MW ceiling to less than 10,000. PE block copolymers with PE MWs in the 2000 to 10,000 MW range are generally poor elastomers. It appears that the hard block MW should exceed 10,000 for good elastomeric properties and preferably should be in the 20,000 to 30,000 range. These disadvantages of anionic ethylene polymerization (syntheses and properties limitations) have contributed to the lack of success until now in developing useful syntheses, materials, and applications based on this technology.